The present invention relates to a control system for a copier, printer, facsimile apparatus or similar image forming apparatus which uses electrophotographic processes.
An electrophotographic copier, for example, performs a sequence of image forming processes, i.e., charging process, exposing process, developing process, transferring process and fixing process, as well known in the art. An exclusive control unit is customarily assigned to each of such image forming processes (e.g. a high tension power source for a charging and transferring process, a lamp regulator for an exposing process, a bias voltage source for a developing process, and a temperature controller for a fixing process). To make the condition of any of the processes variable or adjustable, it has been necessary to apply to the control unit associated with the process a digital signal having bits the number of which matches with the variable range of condition (e.g. four bits for less than sixteen steps or five bits for less than thirty-two steps) or an analog signal produced by converting the digital signal. For example, in the case that a 5-bit digital signal is applied to each of the control units assigned to the charging, exposing, developing, transferring and fixing processes, twenty-five parallel signal lines (twenty-five bits) are required. Especially, a color copier which involves complicated image forming conditions has a variable range which amounts to more than sixteen steps (six bits) and therefore needs more than thirty signal lines. This is disadvantageous not only from the actual mounting standpoint (wiring and spacing) but also from the cost standpoint.
Some modern control systems designed to digitally control a color copier are implemented by a main controller in the form of a microprocessor. The main controller controls various control units which are built in a color copier, i.e., a grid power source for applying a grid voltage to the grid of a main charger, a bias voltage source for applying a bias voltage to a developing sleeve, a power source for applying a transfer charge current to a transfer charger, a lamp regulator for adjusting the voltage applied to a lamp for exposure, and a temperature controller for controlling the fixing temperature of a fixing roller. Since the voltage, current and temperature governed by these control units are variable over several tens of levels and not constant, the main controller usually feeds signals of more than five bits (thirty-two steps) to the individual control units. The result is the prohibitive number of signal lines and connectors which increase the cost and lowers the reliability of operation. Another problem with such a scheme is that the main controller has to supervise a paper feed section, paper transport section, operation section and other various sections as well and, hence, the control program is prohibitively scaled up to bring about bugs and slow execution. When all of the grid power source, transfer charge power source, lamp regulator and temperature controller are implemented by an analog control principle, they become susceptible to noise and their outputs are difficult to control in a variable fashion. Thus, the dilemmatic situation is that a digital control system is not achievable without installing at least thirty signal lines for digital data alone while an analog control system, although needing only one signal line for each control unit, is susceptible to noise. Furthermore, the various control units stated above are traditionally controlled by an analog system and, therefore, cannot adapt themselves to various kinds of loads (specifications, power, input/output characteristics, etc.). Even the controller itself is not operable when the control units are replaced.